Multiple entry threads (i.e., multiple parallel threads on a single shank with each thread having its own entry point at the end of the shank) are used for providing quick engagement between the threaded shank and the opposingly threaded, mating part. The mating part must also have multiple threads and requires a thread pitch equal to that of the multiple threads. Multiple entry threads have a number of applications, including their use to aid the quick replacement of automobile racing wheels.
The conventional method for machining multiple entry threads is to initially produce a single thread and then to machine each additional thread by starting the cutting tool from a backed off position (along the threading axis) whose distance away from the thread entry point is proportional to the thread lead and inversely proportional to the number of threads to be cut. For example, to produce a double entry thread by the prior art method, an initial thread is cut, then the cutting tool is backed off from the initial entry point by one-half of the thread lead (thread lead is the distance between turns of the same thread). The backed off distance is thus added to the length of the threading path followed by the thread cutting tool. This method is effective because the speed at which the cutting tool is moved forward is directly related to the speed of the spindle which is turning the workpiece. As the workpiece rotates, the cutting tool advances to make contact at the desired entry point. This conventional method is more fully discussed herein below in connection with FIG. 1 and in contrast to the method of the present invention.
A significant problem with the prior art method is that clearance is always required at the end of the workpiece to accommodate the backoff placement of the cutting tool. This accommodation cannot always be easily made. In addition, there are the problems of wasted motion and time as the cutting tool traverses the backed off distance on each cutting pass. In effect, the threading pass is made longer for each additional thread although the thread length remains the same.
It is, accordingly, the principal object of the present invention to provide a method for producing multiple entry threads by which the need for tool backoff is eliminated; a method which may be implemented in a numerically controlled system in which the thread cutting tool starts from substantially the same position for each thread.